VNAV Target Cruise Speed Too Low?

[BMFlight]

I noticed that the cruise speeds are pretty low up at cruise when VNAV is on.  For example, flying at FL330 with a cost index of 35 has a cruise speed of .73.  I could be wrong, but haven't noticed the speeds being this low before - usually at around .74 with a cost index of 28.  I am running 1.0.3, and speeds seemed to be fine with the previous versions.

[Morten]

M.74 is normal cruise speed for the classic

[Eddie]

Enroute wind might've influenced performance calculations.

[Alpha Floor]

1 hour ago, Eddie said:

Enroute wind might've influenced performance calculations.

The enroute wind might influence cruising speeds, but that's a second order effect.

The 737 is not a fast airliner. Classics cruise at .74 and NG's cruise at .78

Airbus 320 cruises at .80. 757/767 at .82. 777 at .84 and the fastest of all is the 747 at .86

[Eddie]

1 minute ago, Alpha Floor said:

The enroute wind might influence cruising speeds, but that's a second order effect.

The 737 is not a fast airliner. Classics cruise at .74 and NG's cruise at .78

Airbus 320 cruises at .80. 757/767 at .82. 777 at .84 and the fastest of all is the 747 at .86

Yes, obviously - but OP reported having .74 before the update and .73 after, which could easily be explained by variances within the same CI.

 

Also, the A320 doesn't typically cruise at .8 (I've only ever seen .78 for ECON) and typical cruising for the 767 is .8, 757's a bit slower to my knowledge.

[Alpha Floor]

1 minute ago, Eddie said:

Also, the A320 doesn't typically cruise at .8 (I've only ever seen .78 for ECON) and typical cruising for the 767 is .8, 757's a bit slower to my knowledge.

Thanks. Numbers were off the top of my head

3 minutes ago, Eddie said:

but OP reported having .74 before the update and .73 after, which could easily be explained by variances within the same CI.

I'm not sure I understand this. Changes in CI will obviously change the cruise ECON SPD. But within the same CI, if the pilot doesn't input the wind information into the FMC then VNAV won't care about actual wind while in cruise and won't change the ECON SPD. Correct?

[Litjan]

No, the ECON speed flown is influenced by current wind and adjusts with changing winds.

Jan

 

[Alpha Floor]

Gotcha! Thanks for that Jan!

[Alpha Floor]

By the way, this is a great article on Cost Index and Fuel Strategies, by Boeing's Aero Magazine:

http://www.boeing.com/commercial/aeromagazine/articles/qtr_02_10/pdfs/AERO_FuelConsSeries.pdf